The instant invention concerns a ministencil for repairing and/or replacing a unit and/or group of constituents on an electronic board and its operational system. The said stencil allows to deposit solder paste, flux or conductive adhesive.
It may be found necessary, during manufacturing, after a check or when manufacturing has been completed, to replace a defective constituent or to add one or several missing constituents.
In this case, it is necessary to deposit flux and/or solder paste onto the acceptance areas of the constituents in question.
Depending on the board configuration and layout of the various constituents, access to the acceptance areas in question is not always simple.
The known technique is to use metal micro-stencils either with a central support, or a lateral one on two or three sides.
These metal micro-stencils present several drawbacks. With this kind of stencil it is not possible to come closer than 2 millimeters from another constituent located in the immediate vicinity of the acceptance areas to be treated; even when this is feasible, it is impossible to avoid an overflow of solder paste from the mini-stencil when using a mini-scraper during a screen printing operation.
These micro-stencils use the rigidity of the metal, because they cannot be strained. The tension being nil, these stencils are only very rarely perfectly in contact with the surface of the board, and screen printing is, therefore, imperfect.
To have adequate rigidity with the metal stencil a minimum thickness of the stencil is required. This required thickness or minimal thickness poses a problem for fine step screen printing, the size of the smallest acceptable opening being one and a half times the thickness so as to lift the plugs. Indeed, with the known metal stencils used for this operation, it is difficult to attain a pitch of 0.635 mm.
The stencil in accordance with the invention allows to make the rigidity of the stencil independent of its thickness, allowing to use a pitch thickness of 0.4 mm.
The rigidity required of the metal micro-stencil to obtain adequate stability of the stencil does not allow to model it so as to have it in perfect contact with the circuits to be repaired or completed. Moreover, given that the circuits are already equipped with constituents of various shapes and dimensions, it is necessary to conduct positioning mountings of the circuits on the support table to obtain the stencil and circuit to be parallel.
Unless this is done, a fastidious task, the screen printing is imperfect, i.e. completely defective. Moreover, visual positioning of metal micro-stencils on the acceptance areas which can be undertaken only by way of pinpointing through its opening is particularly time-consuming.
The instant invention aims at overcoming these drawbacks by producing, on the one hand, a stencil combined with a rigidity system independent of its thickness, allowing to deposit flux and/or solder paste, and/or conductive adhesive on acceptance areas of a constituent or group of constituents without the product overflowing outside the stencil, the positioning of the latter being, moreover, simplified by means of its design, and, on the other hand, to combine the stencil with a device for operating the said stencil allowing to position the stencil rapidly and in perfect contact with the board, regardless of the medium of the acceptance areas in question or the composition of the board, any type of constituent being accessible to the stencil in accordance with the invention. The stencil in accordance with the invention is flexible and can be moved in space in accordance with 6 degrees of freedom. Therefore, the stencil meets all of the objectives for which it is designed.
To this effect, the stencil in accordance with the invention allows to deposit solder paste, flux or conductive adhesives on acceptance areas for distributing and/or replacing constituents on an electronic board, is generally characterized by the fact that it comprises:
a flat sheet, of rectangular shape, made of synthetic flexible material or metal, outfitted with transfer openings for distributing a product at a constant amount corresponding to the acceptance areas and the tabs of the constituents;
two of the sides parallel to the sheet are connected to perpendicular tabs to the sheet, rigid and mobile in the position of gripping the sheet onto a support and subjecting the sheet to tension.
the two other parallel sides of the sheet are extended with flexible flaps, a stiffening and jointed fold separating the perforated sheet and each of the flexible flaps, the said fold delimiting the sheet, the flaps freely rotating around the said fold.
In accordance with another characteristic of the invention, the stencil forms an enclosure with a flexible bottom with openings, and opened toward the top, and accessible from the top or two of its sides, or at least one of them.
In accordance with another characteristic, the sheet is semi-transparent or translucent.
In accordance with yet another characteristic of the invention, the flexible flaps orthogonal to the sheet are free with regard to the other sides of the sheet connected to the affixing tabs.
In accordance with yet another characteristic of the invention, the tension unit for putting the stencil under tension, to which, in accordance with the invention, the stencil is affixed by a wing parallel to the sheet of each of the rigid tabs perpendicular to the sheet includes:
a stencil tension axle by spreading or contracting the tabs parallel to the sheet,
a displacement control axle of the stencil support devices, parallel to the sheet for stretching the stencil;
a rotation axle for each tab parallel to the sheet;
a rotation axle for each tab perpendicular to the sheet; allowing to always position and shape the stencil locally as a function of the position of the board, so that it is closely applied to the latter under tension, even when the board is warped. One operational element formed by an articulating arm allows the stencil to reach any position in space.